a) Field of the Invention
The invention is directed to a process for the production of aqueous chlorine dioxide solutions through oxidation of chlorite with oxo acids and/or oxo acid anions having a suitable redox potential in a buffered aqueous medium.
b) Description of the Related Art
Numerous attempts have been made up to the present, without satisfactory results, to kill Legionella in hot water systems in order to reduce potential health risks. Further, conditions exist in industrial cold water systems which favor bacterial growth and which lead to a biofilm on the surfaces of transporting lines which has been countered up to the present by biostatic means. For example, isothiazolone, thiocyanates, quaternary ammonium compounds and chlorine-containing compounds are used for this purpose. A great disadvantage of these biostatic means consists in that the cold water often has CSB/BSB values above the limiting values due to the large amounts used. In fact, only an increase in temperature to about 73.degree. C. and an increase in the flow volume have proven suitable for killing Legionella in water systems. However, there is a risk that after a certain period of time the Legionella which have not been killed will propagate again from the biofilm and encrusted deposits in the water line system, especially from so-called dead strains, and will once again cause lasting contamination of the system. Therefore, nonionic phosphonic acids with hydrogen peroxide (EP 0 540 772 A1) or grapefruit seed extract (EO 0 602 891 A1) were recently proposed for combatting Legionella. However, none of these methods has been successful.
Aqueous chlorine dioxide solutions are promising candidates for the areas of application mentioned above. However, the known processes for industrial production of aqueous chlorine dioxide solutions have grave disadvantages which pose obstacles to the use of chlorine dioxide solutions produced by these methods in drinking water and in related areas.
According to the process in DE-PS 27 28 170, 7 to 21 parts by weight of chlorite, 7.5 to 22.5 parts by weight of hypochlorite and 0.5 to 1.5 parts by weight carbonate are dissolved, in that order, in 35 to 105 parts by weight water. In order to adjust a slightly alkaline pH, 7.5 to 22.5 parts by weight of diluted inorganic or organic acid are added. A stabilizer, especially in the form of a peroxide, is advantageously mixed in beforehand.
According to DE-AS 27 30 883, an aqueous chlorine dioxide solution is produced by acidifying a chorite solution to a pH of about 4 and subsequently raising the pH value to about 7.0 to 7.2 by adding a water-soluble metal hydroxide. This is followed by the addition of carbonate. The teaching disclosed in DE-AS 27 30 883 offers the advantage that the aqueous chlorine dioxide solution remains stable exclusively through the sodium carbonate, so that additional stabilizers in the form of peroxides, for example, can be dispensed with. The stabilized chlorine dioxide solution is said to be storable for long periods of time, that is, months and years, and is suitable particularly for treatment of drinking water.
According to the teaching of the two processes mentioned above, the carbonate serves to ensure buffering in the basic pH range. However, the incorporation of the carbonate stabilizing in the alkaline range leads to a high pH value which is undesirable for various reasons: a high pH promotes the reformation of chlorite ions so that the known chlorine dioxide solutions always contain an unwanted proportion of chlorite ions. Moreover, in the alkaline range, highly toxic and therefore undesirable chlorate can form through various types of reactions. For example, when alkaline hypochlorite solution is mixed with chlorite solution, the oxidation of the chlorite beyond the oxidation stage of the chlorine dioxide leads to chlorate. Further, chlorine dioxide tends in the alkaline range toward disproportionation in chlorate and the by-product chlorite.
The process according to DE-PS 34 03 631 is also directed to the production of a slightly alkaline system based on a chlorite solution. It aims at the production of an aqueous "chlorite" solution which is stabilized by a peroxide compound, modified, and adjusted in the alkaline range. In this process, an aqueous solution with a pH of 3 or less containing sulfate ions is mixed with a peroxide compound which is stable therein, wherein a 0.001 to 0.1 molar concentration of peroxide compound results in the end product. This solution is mixed with an aqueous alkaline chlorite solution in such quantity that there results a pH of greater than 7.0, especially between 7.5 and 8.0. A water-soluble phosphate is preferably added to the resulting end product, wherein a buffer action occurs as a result of the third stage of dissociation of the orthophosphoric acid. This satisfies the requirement that a pH value of greater than 7 is maintained.
Although the known process described above admittedly leads to advantageous uses in individual cases, subsequent scientific tests have shown that the chemical designations connected with it are not applicable and, further, improvements would be desirable. With respect to the incorrect formulas, reference is had to M. Rimpler, W. Regment, D. Pacik, "Balneozoon und Hydroxan, Die Anwendung non halogenhaltigen Sauerstoffkomplexen fur die Balneologie und den Schwimmbadbereich [Balneo-organisms and Hydroxan, The Use of Halogen-containing Oxygen Complexes in Balneology and Swimming Pools]", Forum Stadte-Hygiene 43 (1992) Sept./Oct., pages 226-230. It is shown in this article that chlorine dioxide formed according to the known process is transferred in the presence of chlorite ions to a charge-transfer complex of the formula Cl.sub.2 O.sub.4.sup.- which converts to the tetrachlorodecaoxide anion in the alkaline range by means of oxygen. This anion can also be formed from chlorine dioxide through the action of an oxidizing agent such as hydrogen peroxide. The disadvantage of the products obtained by means of the known methods consists in that they all contain chlorite which is disadvantageously liberated when used, for example, for the treatment of drinking water. In this connection, it must be considered particularly in regard to the treatment of drinking water that legally prescribed limits for chlorite may not be exceeded. The currently applicable maximum value is 0.2 mg chlorite/l.
Oxoferin.RTM. is a known activated oxygen which is stabilized in alkaline medium and embedded in a matrix of chlorite ions. The stabilized activated oxygen is in the form of a solution. A medication containing this activated oxygen can be advantageously used for the treatment of skin damage or for wound healing disorders. Its production is described in EP-A-0 093 875.
In the Olin system, chlorine bleach and a chlorite solution are reacted at a pH of 3.5 to 4. Sulfuric acid is used to adjust the pH. This system is commercially available as Model 350 Dioxolin. This process is carried out in a single mixing process, which leads to uncontrolled processes and complex mixtures. It has the further disadvantage that no buffer is provided by the sulfuric acid.
As was first described in W. J. Masschelein, Trib. Eau 42 (542); 1990, pages 49-52, chlorine dioxide has a toxic effect on Legionella. The chlorine dioxide used for technical purposes in this reference was obtained from chlorite and acetic anhydride. After consumption of the strong oxidizing agent chlorine dioxide, a mixture of this kind, due to the acetate carbon body contained therein, is a nutritional base for other undesirable microorganisms especially in drinking water systems. This fact therefore militates against the use of an aqueous chlorine dioxide mixture of this kind in the foodstuff and drinking water domains, especially since the addition of organics to drinking water has meanwhile been prohibited in many countries (e.g., in the German Federal Republic (TVO)).
Aside from the above-described processes for chlorine dioxide production through oxidation or disproportionation of chlorite, chlorine dioxide can also be obtained by reduction of chlorates. A process of this type is the Mathieson process or sulfur dioxide process. This is a counterflow process in which a solution of sodium chlorate is mixed with sulfuric acid at the head of a reaction vessel and an air-sulfur dioxide mixture is blown in at the bottom. An air-chlorine dioxide mixture can be removed from the head of this reactor. This mixture also contains proportions of chlorine.
In order to reduce any chlorine that is formed, a surplus of sulfur dioxide is used in the process. The addition of chloride ions also increases the yield of chlorine dioxide. A process of this kind is disclosed, e.g., in WO 90/05698.
The Solvay process or methanol process uses methanol for the reduction of the chlorate. Since the reaction speed of this process is lower than that of the Mathieson process, the process must be carried out at a higher temperature. A process of this kind is described, e.g., in EP 0 357 198.
Chlorate can also be reduced to chlorine dioxide by chloride. The resulting chlorine dioxide is always contaminated with chlorine gas. If this contamination is undesirable, the chlorine dioxide is absorbed by an aqueous solution and thus separated from the chlorine. A process of this kind is described in EP 0 106 503.